A tilt rotor or tilt wing aircraft typically employs a pair of rotor systems which are supported at the outermost end of a wing structure and are pivotable such that the rotors thereof may assume a vertical or horizontal orientation. In a horizontal orientation, the aircraft is capable of hovering flight, while in a vertical orientation, the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft.
Currently, tilt rotor/tilt wing aircraft employ conventional fixed-diameter rotor systems which, in the aerodynamic and aeroelastic design thereof, attempt to blend the competing requirements of hovering and forward flight modes of operation. For example, with regard to hovering flight, it is generally advantageous to employ a large diameter rotor to improve hovering performance by lowering disk loading, reducing noise levels, and reducing downwash velocities. Conversely, a relatively small diameter rotor is desirable in forward flight to improve propulsive efficiency by minimizing blade aero-elastic properties, minimizing blade area, and reducing tip speed (Mach number).
Variable Diameter Rotor (VDR) systems are known to provide distinct advantages over conventional fixed-diameter rotors insofar as such systems are capable of successfully operating in both modes of operation. That is, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency.
An example of a VDR system and VDR blade assembly therefor is shown in Fradenburgh 3,768,923 wherein each blade assembly includes an outer blade segment which telescopes over a torque tube member so as increase or decrease the rotor diameter. The outer blade segment includes a structural spar, i.e., the foremost structural element which carries the primary loads of the outer blade segment, a leading edge sheath assembly and trailing edge pocket assembly, which sheath and pocket assemblies envelop the spar section to define the requisite aerodynamic blade contour. The torque tube-member mounts to a rotor hub assembly and receives the spar member of the outer blade segment. The torque tube member, furthermore, functions to transfer flapwise and edgewise bending loads to and from the rotor hub while furthermore imparting pitch motion to the outer blade segment. The resultant torque tube/spar assembly forms a central channel for housing a retraction/extension mechanism. The retraction/extension mechanism includes a threaded jackscrew which may be driven in either direction by a bevel gear arrangement disposed internally of the rotor hub assembly. The jackscrew, furthermore, engages a plurality of stacked nuts which are rotationally fixed by the internal geometry of the torque tube member yet are permitted to translate axially along the jackscrew upon rotation thereof. Furthermore, centrifugal straps extend from each nut and are affixed via a retention plate to the tip end of the spar member. As the jackscrew turns, the stacked nuts are caused to translate inwardly or outwardly, thereby effecting axial translation of the outer blade segment. Systems relating to and/or further describing VDR systems are discussed in U.S. Pat. Nos. 3,884,594, 4,074,952, 4,007,997, 5,253,979, and 5,299,912.
To facilitate mounting of the retraction/extension mechanism to the spar member, prior art variable diameter rotor blade assemblies incorporate a removable tip cap for providing access to the retention plate. While this construction facilitates blade assembly, the resultant stiffness requirements necessary to react the centrifugally-induced compressive buckling loads are maximum at this radial station. This is more readily appreciated by recognizing that the sum of the centrifugal forces of the spar member/outer blade segment culminate at the tip end of the blade assembly. To accommodate the compressive buckling loads, structural augmentation is required at the outermost end portion of the spar member which adversely affects the weight distribution of the blade assembly. That is, the high stiffness and, consequently, weight at the tip end of the blade assembly, requires structural augmentation of the torque tube member to maintain the edgewise frequency design criteria established for the blade assembly. Accordingly, weight penalties are incurred in the torque tube member as a consequence of the additional weight at the tip end of the blade assembly.
Weight penalties at the tip end of the blade assembly can also have adverse effects on the overall weight and complexity of the retraction/extension mechanism. For example, one kilogram (2.2 lbs) of additional mass at the tip end of the outer blade segment produces nearly 6000 N (1,350 lbs) of centrifugal force when the VDR blade assembly is in a fully-extended position. Insofar as centrifugal load is transferred to the rotor hub by the retraction/extension mechanism, the strength and, consequently, weight thereof will increase substantially to react the additional centrifugal load.
In addition to the strength/weight requirements necessary to react the compressive buckling loads, the structural joint required for mounting the removable tip cap compounds the weight-related problems by requiring local strengthening/stiffening of the associated joined components. Furthermore, due to the high centrifugal field at this radial location, the structural joint is a potential source of fatigue failure. Generally, therefore, tip caps must be designed for low weight/small span to reduce the weight and complexity of the structural joint. Competing with this design requirement is the desirability of incorporating an aerodynamic blade tip which may be tapered, twisted or swept to reduce Mach effects, reduce induced and profile drag losses and increase lift coefficient. As such, a large span blade tip may be desirable to effect a smooth thickness or twist transition.
A need therefore exists for providing a mounting arrangement for retention of the outer blade segment which reduces weight at the outermost end portion of the VDR blade assembly, eliminates the requirement for structural joints, and, facilitates the formation of an aerodynamic blade tip.